The invention relates to a method to deal with space charge effects in ion cyclotron resonance mass spectrometers and to devices for applying this method.
Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS), where the ions are trapped by a magnetic field and an electrostatic field in the ICR trap, delivers the highest mass precision and the highest mass resolution among all mass spectrometric methods. The mass precision, however, is strongly influenced by the space charge inside the ICR trap, and trials to control the space charge are of moderate success only.
The operation and function of a conventional ion cyclotron resonance mass spectrometer can be described by using FIG. 1. Ions are produced, for example, by electrospray in a vacuum-external ion source (1). They are introduced, together with ambient gas, through a capillary (2) into the first stage (3) of a differential pumping system, which consists of a series of chambers (3), (5), (7), (9), (11) and (13) and is pumped by the pumps (4), (6), (8), (10), (12) and (14). Ions in the chambers (3) and (5) are drawn in by the ion funnels (14) and (15) and transferred into the multipole ion guiding system (16), in which ions can be either guided through or also be stored. The ions are subsequently transferred through a quadrupole mass filter (17) and through another multipole ion guide (18) that also allows ion storage, and finally via the main ion transfer system (19) into the ICR trap (20), where they are captured and trapped.
The ICR trap (20) usually consists of four mantle-shaped enclosing longitudinal electrodes (21) and of two trapping electrodes (22) with a central hole in each of them. The vacuum system has a laser window (23) in order to allow photo-dissociation experiments in the ICR trap. The ICR trap is located in the homogeneous zone of a strong magnetic field that is generated by superconducting coils in a helium cryostat (24), and has a high constancy in time, as well as a high homogeneity. The magnetic field is aligned parallel to the longitudinal electrodes (21) of the ICR trap. In the ion transfer system the ions can be deflected or diverted to a detector for an external determination of the total ion current. In FIG. 1 such a detector (25) is depicted as an example. Ions can be deflected into the direction of this detector whenever a total ion current measurement is desired.
In an ion cyclotron resonance mass spectrometer ions are radially trapped by the strong magnetic field and perform their cyclotron motions. In order to trap the ions along the magnetic field lines in axial direction, repulsive electric fields are used. They are generated by a DC-voltage (trapping voltage) that is connected to trapping electrodes (usually planar plates), which cover the front end and the back end of the ICR trap. This voltage generates a repulsive potential for ions. The excitation and detection electrodes of the ICR trap are on ground potential in terms of the DC-voltage. Consequently, the ions perform an additional motion in the trap, which is independent of the cyclotron motion, the so-called “trapping oscillation”. For the determination of masses with the FT-ICR mass spectrometer ions are excited to cyclotron orbits using a dipolar frequency-scanned radiofrequency field. During the excitation, all ions with the same mass-to-charge ratio form coherent ion clouds, which induce image currents when they fly near the detection electrodes. These image currents are amplified, digitized and Fourier transformed into a frequency spectrum. From this frequency spectrum, mass values can be determined with a high precision. A mass calibration leads to accurate mass values.
The application of the trapping voltage destroys the ideal conditions for a pure cyclotron motion, which is actually defined in an electrically field-free environment in a homogeneous magnetic field. However, since the trapping field is necessary for the operation of a conventional state of the art ICR trap, the electrostatic field thus created in the ICR trap is tolerated. The axial and radial components of this electrostatic field are dependent on the position inside the ICR cell. As the ions move due to their trapping oscillation and their cyclotron motion (in particular after the excitation by the RF field) in a relatively large volume in the trap, they experience different forces caused by position-dependent electric field vectors.
A common problem of all ion traps is the space charge due to the large number of ions stored in the trap at the same time. The space charge creates an extra electric field in the ion trap additional to the electrostatic trapping field. The electric field of the space charge in the ICR trap interacts with the cyclotron motion of the ions, as well as with the electrostatic field of the trapping electrodes.
In theory, the revolving frequency ωc=2πvc of the cyclotron motion of ions with mass m and charge q in a magnetic field of flux density B is given as the so-called cyclotron frequency ωc=qB/m. The frequency of the trapping oscillation of ions in the direction of the magnetic field axis is given as ωT=√{2qαVT/(ma2)}, where VT is the applied trapping voltage, a the distance between the trapping electrodes and α a geometry constant. The trapping potential also affects the cyclotron motion and changes the cyclotron frequency. The reduced cyclotron frequency ω+ due to the trapping potential is given by the following well known equation [1]:
                              ω          +                =                                            ω              c                        2                    +                                                                                          ω                    c                    2                                    4                                -                                                      q                    ⁢                                                                                  ⁢                    α                    ⁢                                                                                  ⁢                                          V                      T                                                                            ma                    2                                                                        .                                              [        1        ]            The space charge causes a further reduction of the cyclotron frequency to a value of ωR,+ which is expressed by an additional negative term in the square root of the equation [1] above:
                              ω                      R            ,            +                          =                                            ω              c                        2                    +                                                                                          ω                    c                    2                                    4                                -                                  (                                                                                    2                        ⁢                        q                        ⁢                                                                                                  ⁢                        α                        ⁢                                                                                                  ⁢                                                  V                          T                                                                                            ma                        2                                                              +                                                                  ρ                        ⁢                                                                                                  ⁢                                                  q                          2                                                ⁢                                                  G                          i                                                                                                                      ɛ                          0                                                ⁢                        m                                                                              )                                                      .                                              [        2        ]            Here, ρ is the number density of the ions, Gi is a geometry factor of the ion cloud, ∈0 the dielectric constant of vacuum. Expression (2) has to be considered as critical in practice because experiments indicate that not only the number of charges but also the distribution of the charges onto ions of different masses influence the measured ion cyclotron frequency.
A device to avoid the electrostatic trapping field in FT-ICR mass spectrometry is described in U.S. Pat. No. 7,038,200 B2 (E. N. Nikolaev), in which an ICR trap is introduced, that utilizes trapping electrodes with parallel wires (or a pattern of electrodes). In that invention, the trapping electrodes of the ICR trap are connected to an alternating radio frequency voltage in a way, so that the adjacent elements of the pattern (e.g. wires) have different phases. Thus, only ions in the direct vicinity of the trapping electrodes experience a repulsive force and return. The well known term “pseudopotential” provides a vivid description of this effect. In a first approximation, the pseudopotential is a function of the local electric field strength, of the mass and the charge of the ion, as well as the frequency of the alternating electric field. In the following, this pseudopotential will be called “alternating field-induced pseudopotential”.
The utilization of an alternating radio frequency voltage for trapping ions in an ICR trap is however associated with problems, since the mass determination is performed here by measuring radio frequencies of the extremely tiny image currents. From the patent applications US 2006/0226357 A1, DE 10 2004 061821 A1 or GB 2 421 632 A (J. Franzen and E. N. Nikolaev) it is known that there is a further way of operating an ICR trap with a pattern of trapping electrodes, which no longer requires a radio frequency voltage. In this ICR trap the adjacent elements of the electrode pattern situated at both ends of the ICR trap generate electrostatic potentials with opposite polarities. If ions that revolve on their cyclotron orbits approach the trapping electrodes, they cross the inhomogeneous fields of the electrode elements of different polarities. Hence, they experience an inhomogeneous and alternating field due to their motion relative to these electrodes, at which they are reflected back into the ICR trap. In this case the ions experience a repulsive pseudopotential during their flight passing near the electrode elements as a result of their motion. A resting ion would not experience such a pseudopotential. In the following, this pseudopotential will be called “motion-induced pseudopotential”.
Such an ICR ion trap is favorably operated with spoke-like elements of the trapping electrode pattern. The operation with DC voltages of alternating polarity has distinct advantages vs. operation with RF voltages: (1) In the case of DC voltages, there is no lower cut-off mass for the reflection of ions, as in the case of an RF operation. (2) The strength of the motion-induced pseudopotential is proportional to the mass m of the ions, whereas the strength of the alternating voltage-induced pseudopotential decreases with 1/m. Heavy ions are much better reflected by a spoke-like grid operated with DC voltages of alternating polarity.
Such an ICR trap provides in its internal volume an environment that is free of electrostatic fields. Under these conditions, an ion describes a cyclotron motion with an unperturbed cyclotron frequency ωc. However, if more ions are captured in the ICR trap, they cause an additional electrostatic field in the trap due to their own space charge, which again reduces the cyclotron frequency of individual ions. This effect is similar to the effect of the conventional electrostatic trapping field.